lesson 12

.gitignore

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 /*/*.bin
+/*/*.o
 /*/*.swp

12-kernel-c/README.md

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+*Concepts you may want to Google beforehand:*
+
+**Goal: Learn to write the same low-level code as we did with assembler, but in C**
+
+
+Compile
+-------
+
+Let's see how the C compiler compiles our code and compare it to the machine code
+generated with the assembler.
+
+We will start writing a simple program which contains a function, `function.c`.
+Open the file and examine it.
+
+To compile system-independent code, we need the flag `-ffreestanding`, so compile
+`function.c` in this fashion:
+
+`i386-elf-gcc -ffreestanding -c function.c -o function.o`
+
+Let's examine the machine code generated by the compiler:
+
+`i386-elf-objdump -d function.o`
+
+Now that is something we recognize, isn't it?
+
+
+Link
+----
+
+Finally, to produce a binary file, we will use the linker. An important part of this
+step is to learn how high level languages call function labels. Which is the offset
+where our function will be placed in memory? We don't actually know. For this
+example, we'll place the offset at `0x0` and use the `binary` format which
+generates machine code without any labels and/or metadata
+
+`i386-elf-ld -o function.bin -Ttext 0x0 --oformat binary function.o`
+
+*Note: a warning may appear when linking, disregard it*
+
+Now examine both "binary" files, `function.o` and `function.bin` using `xdd`. You
+will see that the `.bin` file is machine code, while the `.o` file has a lot
+of debugging information, labels, etc.
+
+
+Decompile
+---------
+
+As a curiosity, we will examine the machine code.
+
+`ndisasm -b 32 function.bin`
+
+
+More
+----
+
+I encourage you to write more small programs, which feature:
+
+- Local variables
+- Function calls
+- Pointers
+
+Then compile and disassemble them, and examine the resulting machine code.

12-kernel-c/function.c

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+int my_function() {
+    return 0xbaba;
+}